Relating to the testing of insulation.



S. EVERSHED.

RELATING TO THE TESTING OF INSULATION.

i APPLICATION FILED DEC. 30- I9I5- 1,296,176. I Patented Mar. 4,1919.

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RELATING TO THE TESTING OF INSULATION.

APPLICATION TILED DEC. 30. m5.

1 ,296, 76. Patented Mar. 19.

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S. EVERSHED.

RELATING TO THE TESTING OF INSULATION.

APPLICATION FILED 050.210.1915.

1 ,296,176. & Patented Mar. 4,1919.

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APPLICATION FILED DEC- 30, 915- I Patented Mar. 4,191?

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S. EVERSHED.

RELATING TO THE TESTING OF INSULATION.

APPLICATION FILED DEC. so. 1915.

Patnted Mar. 4,1919.

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- Mormy less at a high UNITED STATES PATENT OFFICE.

SYDNEY EVERSHED, or CHISWICK, LONDON, ENGLAND.

RELATING TO THE TESTING OF INSULATION.

Specification of Letters Patent.

Patented Mar. 4, 1919.

Original application filed March 21, 1914, Serial No. 826,372. Divided and thisapplication filed December 1 30, 1915. Serial No. 69,467.

To all whom it may concern:

Be it known that I, SYDNEY EVERsHED, a

subject of the King of Great Britain and Ireland, and residing at Acton Lane Works,

Chiswick, London, W., England, have in-' means for ascertaining the condition of the insulation.

It has been known for many years that the resistance of insulation under ordinary working conditions is usually a good deal voltage than it is at a low voltage, and for that reason it is customary to make insulation tests at some pressure not less than the working voltage, a practice which dates from the introduction of testing apparatus invented by me in or about the year 1889.

I have discovered by experimental research that one of the causes of the lower resistance at higher voltage is the presence of moisture in absorbent insulating materials in the form of drops and thin films which form leakage paths for electric current. When an insulator in that condition is subjected to electric pressure the propelling force known aselectrie, endosinose drives water out of the drops into the films, thereby increasing their thickness and therefore decreasing the resistance of the leakage paths. If the electric pressure 18 gradually increased the insulatien resistance of absorbent materials falls, rapidly at first but more and more slowly as higher pressures are attainedand the relation of voltage to insulation re-.

sistance may be expressed by a smooth curve which is convex to the base line from which resistance ordinates are measured, so long as the endosmose or film eflect is the principal factor in determining its shape or law of curvature. But ultimately, if the increase in voltage is continued, the curve passes through a somewhat ill-defined point, or

rather region, of inflection, and changing;

the direction of its curvature begins to bend I r the insulation is made up of two components 105 downward toward zero resistance, the downward 'cunvature rapidly increasing until actual breakdown occurs. The complete characteristic curve, as it may be called, consists voltage,

therefore of two parts of opposite curvature ,55

Joined at a point or region of inflection, the first part indicating the film efi'ect already described, and the second part indicating the growth of a dangerous mode of leakage which ultii nately ends in a breakdown of the insulation. These two significant parts of the whole curve may be conveniently referred to as the film curve and the breakdown curve.

. The law expressing the shape of the film 5 curve is much the same for all absorbent materials, provided they do not contain more moisture than they can harbor in the form of drops and thin films; and the leakage which takes place under that condition is not of. a dangerous character, since it merely consists in each film path carrying a current which is well withinits capacity as a conductor. Changes'in the number of films acting in parallel of theinsul-ation, nor do they alter the law of the curve, although an increase or decrease in the number of film paths necessarily lowers or raises the general-level of the curve.

the first part 'of the characteristic ,curve indicates conduction by films, the general level of the curve-high or low insulation resistance-1s not a matter" of much importance.

On the-other hand the presence of an ex- 85- cessive amount of moisture, particularly when it is in the form of local accumulations of water in bulk, is apt to produce a dangerous conditionin consequence of the exunder such conditions if the excess of water is not detected and removed before the conductors and insulating materials have been seriously damaged. But leakage. conduction of thisv kind follows Ohmslaw, that. is to 95 resistance does notvarywith the and the characteristiccurve would indicate this. law by following a straight line parallel with the base line. Hence the say, the

shape of thefirst part of the characteristic 100 curve enables safe anddangerous modes of leakage through insulating materials to lee-readily discriminated Again, those unumerous cases in which in series, one an absorbent material, and the other a non-absorbent substance which has a constant resistance, the shape ofthe first do not affect the safety Hence so long as the shape of tensive electrolytic action which takes place ing pressure.

part of the characteristic curve will be the resultant of the two corresponding modes of conduction. Hence the failure of either component will be accompanied by a change in the shape of the curve, which will assume the shape of a normal film curve if the nonabsorbent insulator breaks down, or will lose working and so long as the working-voltage does not carry the characteristic curve beyond that point the insulation 'is not in danger.

In these and the-first time a method of insulation diagnosis, and the present practice of testing insulation may therefore be usefully ampli-- fied in the light of my discovery.

The invention broadly consists in apparatus for measuring and testing insulation resistance (or conductance) specifically adapted for the determination and investigation of the whole or any significant part for example two properly chosen points of thecharacteristic curve which expresses the relation between the voltage applied to any electric system or appliance and the resistance (or conductance) of the insulating materials of the system or appliance.

The invention also consists in the improved means relating to the testing of insulation herein described.

Much of the utility of ,thismethod of diagnosis consists in comparing the law or shape of different characteristic curves obtained either from the insulation of an elec-' tric system or appliance at different times under varying natural conditions, or from different electric systems or appliances or circuits. 4 v v 1' In carrying out the invention the testing apparatus is arranged and adapted to facilitate such comparisons by reducing the re sistance ordinates of all characteristic curves to numerical scales which are convenient for comparison preferably so that all curves shall have one coerdinatepoint in common, which is preferably the initial point of the curve corresponding with the minimum test- By this procedure all other points on the curves are expressed by their ratios or percentages with regard to the common point. To carry out this part of the invention the resistance measuring ino I other ways the characteristic curve whether drawn with resistance ord1- nates or conductance ordinates, provides for strument or appliance is modified by the addition of a device adapted tovary the sensibility of the instrument continuously or substantially so, over a considerable range, so that upon making the first test to determine the initial or common point, the

indication-of the instrument may be adjusted until it registers one hundred or one.

thousand or ten, or unity or some other decimal multiple or sub-multiple, of a hundred. When this adjustment has been made the resistance ordinates of the other testpoints on the'curve will evidently be indicated as percentages of the initial resistance ordinate and the law of one curve maybe readily compared with that of any other curve without the necessity for any calculations, and in many cases without even going to the trouble of plotting the characteristic curves.

In carrying out the invention the measuring instrument or, appliance may be of any. known type suitable for the measurement of insulation resistance or insulation conductance, but althoughthe properties of insulators may be expressed in terms of either resistance or conductance, it has so long been the custom to regard resistance as the criterion of insulation that in describing the invention in detail it will be assumed that the testing apparatus'is required-to determine characteristic curves in terms of The necessary testing pressures may be provided by a battery or alternatively by a dynamo or a magneto generator of direct current type, either hand driven or power driven as may be convenient, .and adapted by meane of multiple voltage devices to give the required series of testing pressures. A series of from ten to twelve pressures, suitably graded, will generally enable the complete characteristic curve to be investigated.

on drawn up to the breakdown point if the maximum pressure-of the series isfsuificient for the purpose. A series of six to eight pressures will generally suffice to draw the first part of'the characteristic curve from about 50 volts up'to the point of inflection and if the sole object is to determine the position of the point of inflection with respect to some assigned pres'surethe working voltage of the system for example then a series of three pressures will be' enough. Any known principle either alone or In combination may be-used for/obtaim mg the multiple voltages required by my method of testing, as for example by a subdivided battery, or by varying the speed of a dynamoyor by varylng the induction or the number of effective turns of wire in the. armature of a dynamo, or by a number of windings on a single armature each connected to one of a corresponding number of commutators, or by a multiple armature each element having its appropriate winding.

In order that my invention may be better understood it will now be described with reference to the accompanying drawings in which Figure 1 is a diagram of a complete apparatus for investigating the whole or any part of the voltage-resistance curve of insulation;

Fig. 2 is a modification of the same in which the indicating instrument is an ohmmeter;

Fig. 3 is another modification in which the electromotive forces generated by three dynamo armatures are combined to form a series of testing pressures by means of a permutation switch;

Fig. 4 is another modification of the same in which the speed of the generator which provides the testing current isvaried by -means of a change speed gear in order to give a series of testing pressures;

Fig. 5 illustrates apparatus for carrying out the improved method arranged in a box for portability; A N

Figs. 6, 7 and 8 show a modified form of multiple voltage generator;

In Figs. 3, 4, 5, and 6 the armatures of the multiple voltage generator are alone shown, the field magnets and pole pieces being omitted in order to avoid confusion.

In Fig. 1, E is a source of direct current adapted to give a suitable series of testing pressures, P is a multiple point switch.

adapted so as to apply any of the series of pressures to the insulation resistance W which is to be investigated. G is-an indicating instrument adapted to show the value of the resistance under test, and A and B are two parts of a variable shunt to the instrument coil 0, by means of which the initial reading of a series may be adjusted to some convenient initial or common point.

T T are the terminals which serve to connect the testing apparatus to the insulation resistance which is to be tested.

In Fig. 1 the indicating instrument G is shown by way of example as a galvanometer, e'being the galvanometer coil, d the index and F the scale.- To obtain readings which are proportioned to .the resistance of the circuit the scale is shown divided and figured in inverse proportion to the current traversing the coil 0, and to adjust the initial reading of a series to a common point the coil 0 is connected to a variable shunt which may conveniently be of the Ayrtonadjustments the shunt is divided into two parts R and 1' each sub-divided'into a convenient number of sections and controlled 'by a multiple point switch. For vconvenience the resistance 1" may be equalto a single section of the larger resistance R. To maintain the resistance of the shunted galvanometer at an approximately constant value the switch A is provided with a second switch arm which controls a series of make-up resistances m m, and as a universal shunt gives the maximum resistance to the shunted galvanometer (i. c. to the combined galvanometer coil and the part of the resistance'in parallel with it) when the current divides equally between the galvanometer coil and the shunt, the make-up resistance will be reduced to zero at that 'point. A resistance a of large value is added in series with the shunted galvanometer in order to avoid excessive currents when the resistance W happens to be very low or to break down under test.- The value of a may easily be made ten or even one hundred times as great as the resistance of the shunted galvanometer so that a comparatively rough adjustment of the make-up resistance m m will be sufficient.

In order that the galvanometer G may indicate in the same resistance units throughout the whole series of testing pressures, the voltage switch P is mechanically coupled to a multiple-point shunt switch S which controls a number of resistances k 70 70 etc., which serve as a series of shunts on the combined resistance 'formed by the resistance a and the shunted galvanometer.

oned from the formula Resistance of shunt 7c (a-l-g) (n1),

where g is the constant resistance of the shunted galvanometer and n is the ratio of the testing voltage (for which the shunt k is required), to the initial voltage of the ascending series of testing pressures. In Fig. 1 the source of testing pressure E is The value of each of these shunts is reckshown by way of example as a battery subdivided into eight equal sections and controlled by the eight point switch P. Hence the shunt switch S also has eight positions and at the first point, corresponding with I the testing pressure of a single section of the battery, the switch S stands at a blank contact so that the whole of the testing current traverses the circuit formed by the galvanometer Gand the universal shunt A, B. In this example the values of the shunts I0 70 70 70 must evidently be (a-l-g), tfl -l-t b tW-l-Q) +9)- It is sirable that the total resistance of the current circuit of the testing apparatus should be constant notwithstanding the efiect of f the shunts introduced by the'switch S, and with this object another switch M-is added to control a series of make-up resistances g.

In Fig. 1 the apparatus is shown with the initial voltage of the series applied for testing the insulation represented byv W, andthe common point shunt A B is set for the minimum shunting effect. When so set the scale F may conveniently be arranged to read direct in megohms, and in'that case the pointer d indicates 32 megohms.v 'To bring the pointer to the required common pointin this example the scale division marked 100 would naturally-be chosen as'the initial or common point-the shunting effect is gradually increased by moving the contact arms of the shunt' A B in the direction of the arrows, first obtaining a rough adjustment by means of A and next making a fine adjustment by means of B until the pointer stands at the scale reading 100. To facilitate the accurate setting of the pointer to the desired initial reading, a small continuously adjustable rheostat may be introduced as indicated at N. I Having set the galvanometer reading to the common point the voltage is then increased step by step by turning the voltage from other insulation or from the same insulation under different conditions.

Wvlth this apparatus the insulation resistance of (V at any desired voltage of the series may be obtained in megohms at any time by merely restoring the common point shunt A B to the initial position of minimum shunting efiect.

In Fig. 2 the indicating instrument G is an ohmmeter, 0 being the current coil, and

p the pressure coil. The coarse adjustment to a common point is efl'ected by a universal shunt A on the current coil, and the fine. adjustment may be effected by similar means to those shown in Fig. 1, or preferably by forming a part of the resistance 17 (which is in series with the pressure coil as 'an adjustable rheostat B. In'Fig. 2 the"adjustments A and B have already been efl'ected and the ointer d .is shown standing at the scale division marked 1 preparatory to raising the testing voltage step by step to ob- .tain the required series: of resistance ordinates. In this example the switches S and M are dispensed with because the substitution of an ohmmeter in place of a galvanometer as th indicating instrument renders it ucllinecessary to make any provision of the In either case by whatever means the step by step adjustments of the measuring instrument are made to correspond with the pressure it is convenient to have the adjustment devices geared to or interlocked with the devices which eflect the changes in testing pressure, So that corresponding adjustments and. voltage changes may be carried out either by the movement of one handle or by the coordinated movements of two interlocked handles.

The measuring instrument may conveniently have the scale so divided and figured that it gives direct eadings in ohms or megohms when the instrument is arranged to givethe maximum sensibility, as for example when the shunt for continuous variation of sensibility is set for the'minimum shuntring effect.

In Fig. 3 the testing pressure E is provided by three dynamo or magneto armatures X Y Z, so coupled together mechanicallypreferably by mounting them on one axle-as to constitute a multiple-voltage testing'generator. This generator which may be driven by hand or by power, is adapted to run at a constant speed either by means of a centrifugal clutch or. other known means such as a constant speed motor. The three armatures are so wound as -to' give three component pressures which when added algebraically, will give a num ber of suitable testing pressures. For example if the initial pressure is to be 6 volts when armature X 'shouldbe wound to give e volts, and to obtain the greatest number of testing pressures from the three armatures," Y and Z must be wound to give 36 volts and 9e volts respectively.

The armatures X Y and Z are connected electrically to the three members P P P of a triple permutation switch. Each of these members consists of a multiple point double-pole switch; P having 13 points to' each pole, P having 5 points to each pole; and P having 2 points to each pole. Insuconnect any pair of points with the slip rings H L, arefixed to androtate with the axles O O O and these axles are mechanically coupled so that they all rotate together in the same sense and through equal angles. 1-20 For example the three members of the permutation switch may be mounted one above the other 'sothat the insulated contacts 7:. 2' may be fixed to an axle which is common 'to all three members. of the switch'points are such that in'the case of the. member P the potential difference between the slip rings Hand L is alternately +e, -e, zero, +6, -6, zero, and-so .lated sliding contacts h i which serve to The internal connections on .as the axle O is rotatedin a clockwise direction from point 1 to point 13. During these step by step changes the potential difference between the slip rings of the member P is as follows :zero, +3e, +36, +3e,

thirteen different ways as set forth in the following table In Fig. 3 a triple permutation switch is shown in a simple form in order that the principle by which several component pressures may be added to give a series of voltages may be readily understood. Permutation switches embodying the same principle may be made to add two, three, four or more component pressures giving a series of 4, 13, 40 or more voltage steps. The mechanical construction and the disposition of the vari ous parts of such switches may evidently be varied widely without departing from the principle of algebraic addition.

By way of example the permutation switch in Fig. 3 is shown in connection with,

a common point-adjuster A B .and anohmmeter G- the diagram of connections being similar to Fig. 2. The insulation resistance W, under test, is that between the commutator and windings of an armature and the.

armature axle.

In Fig. 4 the testing voltage is supplied from a variable speed generator E which is driven from the shaft 12, through a centrifugal constant speed clutch 13 andchange-speed gear J. Seven pairs of gears are shown suitably gaduated as regards velocity ratio, and by way of example these gears are shown as actuated by means of seven friction clutches 1, 2, 3,

7 of the magnetic type. These clutches are controlled by a multiple point switch P, cur rent being led to the exciting coils of the clutches by means of slip rings and brushes as indicated by 9 and 10. Current for the magnetic clutches may be derived from any a 7 suitable supply, and in the absence of any otheravailable source. a small dynamo may be provided for the purpose and driven-at.

a constant speed, preferably by mounting its armature 8 upon the constant speed shaft of the change-speed gear. The connections of the galvanometer indicated correspond to Fig. '1.

Other known forms of change-speed gear may be employed preferably of the types which are actuated by friction clutches, and when the indicating instrument G is a galvanometer, the mechanism Which actuates the clutches should be mechanically connected to or interlocked with the voltage shunt switch S and make-up switch M.

fected in the simplest possible manner when magnetlc frictlon clutches are used.

For portability and convenience in use,

a multiple voltage generator and a measuring instrument'with or without any of the various devices herein described for facilitating the process of testing by my method, may be mounted in one box or case or the generator and any of the devices may be so mounted or again the instrument and any of the devices may be similarly mounted.

One such arrangement is illustrated in plan in Fig. 5. The testing generator E shown in this drawing is of the three-component type and is arranged for hand driving by means of a winch handle 15. The necessary'high speed is obtained by suitable gearing 14 interposed between the winch axle and the axle 12. One end of the axle 12 carries the friction drum of'a constant speed centrifugal clutch 13, by which means the three armatures X Y and Z are driven at a constant speed whenever the slipping -speed of the clutch 13 is exceeded. Since the armatures Y and X are only required to generate one-third and one-ninth the pressure generated by Z they may be suitably graduated in size, and to avoid a long span between the bearings of the armature axle, the armature X may be overhung outside the end bearing. The carrying box 16 may be so proportioned as to provide space for the indicating instrument G at one end, and for This interlocking connection is of course efof testing pressures described in connection with Figs. 1 to 5 may be modified in various ways. For example when the armature of the testing generator is wound with a number of separate coils each connected to one of a number of distinct two part commutatorsf (an arrangement commonly used in generators for insulation testing) then a seri-of voltages may be readily obtained from the one armature by connecting a number of I switch, the switch being mounted upon the armature axle and operated by suitable.

A multiple voltage armature conmeans. structed on this principle is illustrated in Fig. 6 where E represents an armature wound with four separate coils connected severally to the four two part commutators 30,31, 32 and 33. Each coil has one end permanently connected to one segment of its commutator, the other commutator segment being connected to the contact arm of a multiple point switch P as indicated in Fig. 8. At suitably graded intervals in the course of the winding 29, tappings are brought to the contact studs of the switch P so that the whole or any prearranged portion of the winding may be connected to the commutator. In this way each armature coil may be made to provide a series of pressures, and by connecting the brushes of the commutators 30, 31, 32 and 33 in series, as indicated in Fig. 6, the separate pressures of the four coils are added together to provide the testing voltage.

There being four armature coils, the complete switch P in Fig. 6 will be composed of four multiple point switches as shown in end-elevation in Fig. 7. The four insulated contact arms 17 are mounted upon a sleeve 18 which is free to rotate on the armature axle 19. Guide pins 22, which project inside an extension of the switch sleeve 18, engage in spiral slots 21 formed on the outside of an operating sleeve 20. The operating sleeve can slide to and fro on the armature axle but is prevented from turning around on it by a feather 23 which engages in a keyway inside the sleeve. The operating sleeve 20 can be set at will to any oneof a number of positions along the armature axle by means of the fork lever 24 which can be retained in the desired position by anotched sector 25 or 7 equivalent device. By these means, or thelr equivalent, the longitudinal motion given to the sleeve 20 is converted into a rotational movement of the contact arms of the switch P and hence the'switch can be set to any desired position notwithstanding the rotation of the armature.

In Figs. 6, 7 and 8, 6-way switches are shown, by way of example, corresponding to a series of six testing pressures. Similar switches may be made for a larger number of ways provided the overall diameter of the complete switch can be sufliciently enlarged to admit of the necessary addition to the number of contact studs.

The invention is not confined to the particular arrangements described, but may be modified in various'ways without departing from the underlying principle of. diagnosis. For example if it is desired to carry out the invention by means of voltage-conductance curves instead of voltage-resistance curves the measuring instruments would be modified to indicate conductance,.units. In the case of a gal-vanometer the modification would consist merely in dividing the scale in direct proportion to the current; 1n the case of an'ohmmeter, the scale'would be divided in inverse proportion to the resistance and figured in reciprocal ohms or other unlts of conductance. A Wheatstone bridge or a diffcrential galvanometer would need no modification except in the rheostats in the adj ustable third arm, which would be composed of a number of unit resistances adapted so that any desired number could 'be connected up in parallel to give any total value of conductance: These and other similar inversions either necessary or convenient for the purpose of convertin resistance measuring apparatus into 00nd uctance measuring 'appa-- ratus will be readily understood by those who proceed by ascending voltages from the lowest required testing pressure upward step by step to the highest pressure of the required series, in order that hysteresis effects, which are a marked feature of conduction through absorbent insulators, may be avoided.

Having now described my invention what I claim as new and desire to secure by Letters Patent is 1. Apparatus for determining any significantpart of the characteristic curve of insulation comprising in combination a measuring instrument whose scale readings are dependent on the current traversing. it, a variable shunt in parallel with the measuring instrument adapted to vary the sensibility of the measuring instrument so that its scale readings may indicate ratios with respect to a convenient fiducial value, variable make-up resistances to maintain the resist-' with the measuring instrument, a source of electromotive force givingthe required series of testing pressures, a shunt of selective resistances in parallel with the said shunted instrument and said resistance in series therewith, a switch for simultaneously selecting the value of the electromotive force and the appropriate selective resistance of the shunt, and make-up resistances to maintain the total resistance of the testing circuit constant.

2. Apparatus for determining any significant part of the characteristi curve of insulatlon comprising in combination an ohmmeter, a variable shunt to the current coil of the ohmmeter, a resistance in series with the pressure coil of the ohmmeter, variable make-up resistances to maintain the resistance of the shunted ohmmeter constant, a

resistance of relatively large value in series with the current coil of the ohmmeter, a source of electromotive force adapted to give the required series of testing pressures, and a switch for selecting the value of the electromotive force.

3. Apparatus for determining any significant. part of the, characteristic curve of insulation, comprising in combination a measuring instrument whose scale readings are dependent on the current flowing through it, means for Varying the sensibility of the said instrument so that in a series of tests of a specimen under different voltages its readings may be expressed as the ratios of a convenient fiducial value, a multiple voltage generator adapted to give a plurality of voltages, and a permutation switch adapted to select from the said generatorthe required series of testing pressures.

4. Apparatus for determining any significant part of the characteristic curve of insulation, comprising in combination a measuring instrument whose scale readings are dependent on the current flowing through it, meansfor varying the sensibility of the said instrument so that in a series of tests of a specimen under different voltages its readings may be expressed as the ratios of a convenient fiducial value, a variable speed generator, driving means, a centrifugal constant speed clutch on the said driving means, change speed gearing between the said clutch and the said generator, a switch and electro-magnetic means for selecting the required speed ratio of the change speed gearing to givethe required speed and testing pressure to the said generator.

5. Apparatus for determining any significant part of the characteristic curve of insulat1on-,compr1sing in combination a measuring instrument whose scale readings are dependent on the current flowing through it,

means for varying the sensibility of the said instrument so that in a series of tests of a specimen under different voltages its readings may be expressed as the ratios of a convenient fiducial value, a variable speed genera-tor, driving means, a centrifu a1 constant speed clutch on the said driving means, change speed gearin with a series of speed ratios between said 0 utch and the said generator, friction clutches for selecting the required speed ratio of the change speed gearing, and a switch and electro-magnetic means for selecting the required speed ratio of the change speed gearing to give the required speed and testlng pressure to the said generator.

6. Portable apparatus for determining any significant part of the characteristic curve of insulation, comprising in combination a measuring instrument whose scale readings are dependent on thecurrent flow ing through it, means for varying the sensibility of the said instrument so that in a series of tests of a specimen under different voltages its readings may be expressed as the "ratios of a convenient fiducial value, a multiple armature generator adapted to give a plurality of voltages, a permutation-switch adapted to select from the said generator the required series of testing pressures, driving means, a constant speed centrifugal clutch between the said drivingu means and the armature shaft of the said generator, and a carrying box in which the aforesaid apparatus is contained.

In testimony whereof, I have afiixed my signature.

SYDNEY EVERSHED. 

